In recent years, human therapeutics have expanded past traditional small molecule drugs and into the realm of biopharmaceuticals. The discovery of novel proteins and peptides has led to the development of numerous protein and polypeptide biopharmaceuticals. Unfortunately, proteins and polypeptides, when utilized as therapeutics, often exhibit properties that make them extremely difficult to formulate or administer, such as short circulating half lives, immunogenicity, proteolytic degradation, and low solubility. One approach for improving the pharmacokinetic or pharmacodynamic properties of biopharmaceuticals is the conjugation to natural or synthetic polymers, such as polyethylene glycol (PEG). The covalent attachment of PEG to a therapeutic protein can provide a number of advantages, such as (i) shielding antigenic epitopes of the protein, thus reducing its reticuloendothelial clearance and recognition by the immune system, (ii) reducing degradation by proteolytic enzymes, and (iii) reducing renal filtration.
Much effort has been spent on the development of polymer derivatives for coupling to biopharmaceuticals such as peptides, and in particular, on the development of polymer derivatives for coupling to reactive amino groups of proteins. Such polymer derivatives are referred to as ‘electrophilically activated’, since they bear electrophilic groups suitable for reaction with nucleophiles such as amines. Examples of such PEG derivatives include PEG dichlorotriazine, PEG tresylate, PEG succinimidyl carbonate, PEG carbonylimidazole, and PEG succinimidyl succinate. Unfortunately, use of these particular reagents can result in one or more of the following: undesirable side reactions under the reaction conditions necessary to effect coupling, lack of selectivity, and/or the formation of weak (i.e., unstable) linkages between the biopharmaceutical and the PEG.
In an effort to overcome some of these problems, many new or “second generation” electrophilically activated PEGs have been developed, such as PEG propionaldehyde and PEG acetaldehyde (see, for Example, U.S. Pat. Nos. 5,252,714 and 5,990,237, respectively). Aldehyde derivatives are particularly attractive reagents for coupling to proteins and other biomolecules, since aldehydes react only with amines (i.e., are selective in their attachment chemistry). The above-mentioned reagents offer many advantages: they can be prepared to avoid the problems of PEG diol contamination, are not restricted to low molecular weight mPEG, form stable amine linkages upon coupling, and are selective. Although the above noted derivatives offer many advantages over first-generation PEG reagents, the Applicants have noted some particular drawbacks of these aldehyde reagents, making them less than ideal in certain instances.
More specifically, the Applicants have recognized, in their extensive work with these reagents, that PEG acetaldehyde is very unstable, particularly in basic media, and is difficult to isolate due to excessive salt formation resulting from neutralization of the reaction mixture. In particular, PEG acetaldehyde is very susceptible to dimerization via aldol condensation. PEG propionaldehyde, while a much better reagent in terms of its stability, possesses some disadvantages due to side reactions that can occur during its preparation, making it difficult to obtain the PEG propionaldehyde product in high purity.
More specifically, the Applicants have found that when preparing PEG propionaldehyde in situ from its precursor PEG aldyhyde hydrate, product yields are generally only about 50%, due to an elimination reaction that consumes a significant portion of the acetal reagent. Although an improved synthetic route for the synthesis of PEG propionaldehyde can be employed, i.e., via base-catalyzed reaction of 3-hydroxypropionaldehyde diethyl acetal with PEG mesylate, the Applicants have discovered that this reaction route also leads to an elimination side reaction that produces significant amounts of PEG vinyl ether, which is unstable and produces difficult-to-remove the parent dihydroxy PEG (also referred to as PEG diol). Consequently, the yield of this reaction is generally less than about 85 to 90%. Moreover, using either of the above-described PEG propionaldehyde syntheses requires hydrolysis of the acetal intermediate at very low pHs, e.g., at pHs of 2 or lower. Hydrolysis at such low pHs is undesirable due to the large amounts of base necessary to neutralize the reaction mixture to pHs suitable for conjugation. Additionally, coupling PEG propionaldehyde to a protein at basic pHs can be problematic due to formation of significant amounts of acrolein (resulting from a retro-Michael type side reaction), which is quite difficult to remove. Formation of such undesirable side products necessitates extensive purification to obtain a pharmaceutical grade product.
Thus, there exists a need for improved electrophilically activated polymer derivatives for conjugating to biologically active molecules and surfaces, particularly polymer derivatives that (i) are selective in their coupling chemistry, (ii) can be prepared in high yields and in few reaction steps, (iii) are stable over a wide range of pHs, (iv) can be readily isolated, (iv) can be prepared in high purity (i.e., substantially absent polymer-derived impurities and side-products, and (v) overcome at least some of the drawbacks of known polymer derivatives such as those described above.